Electronic devices which can transmit a modulated electromagnetic signal that can be detected by a corresponding reader are widely used for identification and tracking of items, particularly for supply chain management, or item level product tracking. Such devices, which will be referred to as RF tags, include RF devices electrically connected to an antenna that is tuned to a particular frequency of operation, such as UHF (Ultra High Frequency) RFID (Radio Frequency Identification) tags, and Microwave RFID (Radio Frequency Identification) tags. The tags may be passive tags, which interact with reader by signal backscattering, semi-passive tags, which are powered by either their own power sources or external electromagnetic waves, or active tags, which are powered by its own power sources.
One disadvantage of these RF tags is that the reading range is reduced significantly when these tags are directly placed on a metallic surface. In a common case, the tags cannot be read. When the tags are placed close to the metallic surface at a distance less than λ/4, λ being the wavelength at the frequency of operation of the RF tags, the RF tags' reading range will also be reduced. When RF tags are placed near or directly on a metallic surface, the surface current on the metallic surface will degrade the efficiency of RF tags. Therefore, the tracking of metallic objects is very difficult for UHF RF tags and Microwave RF tags, and more typically, passive RF tags.
Another disadvantage of these RF tags is that the RF tags are commonly designed for tracking RF friendly materials, such as paper, and low permittivity plastics. When these tags are placed on RF unfriendly materials, such as high permittivity materials, magnetic materials, and liquid, the reading range will also be reduced, more typically to an unacceptable level.